Food, when frozen, can be preserved over a prolonged period by retarded biochemical reaction along with suppressed action of microorganism under decreased activity of water in the food owing to low temperature and freezing of the water contained in the food. The limit temperature for multiplication of microbe is about -10.degree. C., and that of yeast is about -18.degree. C.
It is said, therefore, that food is unlikely to be rotten or decomposed by self digestion when refrigerated under -18.degree. C. (deep frozen state).
According to the Japanese Agricultural Standard, freezing method of processed food article such as hamburger and meatball should be such that the food article are rapidly passed through a temperature range from -1.degree. C. to -5.degree. C. which is the maximum ice-crystal formative temperature range in which most part of the water in the food freezes, to be frozen to -18.degree. C.
Water contained in food comprises free water which can move freely and easy to be frozen and bound water which is hard to be frozen. The latter is the water in hydrated molecule of protein, glycogen, etc. and the higher the degree of hydration is, the harder it is to be frozen. So it is the free water in food that forms ice-crystal when the food is frozen. When food is cooled to freezing point, ice separates as crystal in the water solution in the food. When the food is further cooled below freezing point to the maximum ice-crystal formative temperature range (temperature range from -1.degree. C. to -6.degree. C. in the case freezing point of the food is -1.degree. C.), ice-crystal grows in this temperature range, which causes the destruction of cell membrane. Therefore, it is desired that minute ice-crystals are separated by quick freezing in which the food is cooled passing through the maximum ice-crystal formative temperature range in short time so as to restrain the physical destruction of cells to the minimum.
It is also said that frost damage does not occur in the case where freezing is performed in a way in which, as even in the case of the said quick freezing a lot of small ice-crystals are formed in and outside of a cell and destruct the cell, this phenomenon is prevented by restraining initial freezing speed to a degree the separation of ice-crystal in the cell does not occur and shifting to quick freezing after the outside of the cell is solidified with ice-crystals.
For the operation without frost damage like this, it is necessary to know accurately an ever-changing temperature of food during freezing and to control freezing through programmed control responding to the change of the temperature.
Also, the uniformization of uneven temperature in a food article from the surface to deep part caused by quick freezing is necessary to be performed by interposing a temperature uniformizing freezing process between the said quick freezing and deep freezing (freezing to a temperature under the maximum ice-crystal formative temperature, for example, to a neighborhood of -20.degree. C.), which temperature uniformizing freezing is possible by the change of freezing medium and freezing condition.
In this case also accurate understanding of food temperature and programmed control of freezing based on the understood measured food temperature is necessary.
It is said that, particularly in the freezing process of a cooked food article, the physical damage of fleshy or pulpy substance should be minimized by obtaining finer ice-crystal in quick freezing to minimize the physical and chemical influences to the composition of the food and then storage in low temperatures under -18.degree. C. with small deviation of .+-.2.degree. C. is necessary lest the growth of ice-crystal does not proceed fast during the storage.
In an individual freezing in which small sized food articles are individually frozen, it is necessary to measure the surface, center, and balanced temperature of each food article, to draw up their freezing curves, to freeze under an appropriate condition of freezing environment temperature, freezing speed, etc., and to check food temperature after freezing.
In unfreezing of food, rapid passing through the maximum ice-crystal thawing zone by rapid unfreezing is required, in the point of view of biochemical and enzymatic reaction, to keep the food temperature under -10.degree. C., possibly under -5.degree. C., until the food temperature is raised to 0.degree. C. That is, also in this case, an unfreezing with high temperature at initial stage and then a programmed unfreezing with low temperature is required.
It is important to efficiently produce with low mortality homogeneous and high quality frozen food articles, that various states of the food such as temperature, temperature decreasing speed, freezing speed, completion of freezing, shape, size, composition, and in the case of continuous cooling/freezing/heating apparatus, arrangement on the transfer belt and transfer speed, are grasped and the most suitable and efficient operation for freezing the food is performed.
By the way, hitherto, the measurement of the temperature of a food article in freezing or unfreezing process has been performed by measuring the temperature of the surrounding air, water, or brine which comes in contact with the food article, or of the surface of the food article by allowing a sensor to contact the same, or of the inside of the food article by thrusting a sensor into the same and thus the temperature of a food article has not been grasped as the whole.
In the case of measurement by thrusting a sensor, the measured food article can not be used as a product, and in the case of measurement by contacting a sensor it is unhygienic.
Further, according to the conventional method, measurement of temperatures at various parts of a number of food articles have been practically impossible and so measurement has been performed on selected parts of a small number of sampled food articles and inferences have been made from the freezing environment such as the temperature, speed, direction of the cooling air, and freezing period. The control of the operation of a freezing apparatus has been done through feedback of the data of the freezing environment not that of the data of the temperature and degree of freeze of the food articles.
In the mean time, from now, to respond to the demands from user side such as PL Act, HACCP (Hazard Analysis and Critical Control Point), cost down, and differentiation of product with high added values, the quality control and operation control with high accuracy will be indispensable. However, by conventional method, a variety of sensors and measurement devices such as; for example, contact and/or insert temperature sensor; radiation surface thermometer; chemical and physical analyzer of sampled piece of food; X-ray or radiologic, magnetic, supersonic, photoelectronic devices; image sensor; touch sensor; have been used in accordance with increasing measurement items, and there have occurred problems such as the complication of apparatus and its operation, increase in the number of samples, and increase of work for caring sensors.
The most crucial point is the problem of hygienic quality of food article due to contacting of sensors with the food article.
For example, when measuring the temperature of a food article with a thermistor and so forth, the tip of the sensor must be inserted into the food article and so the sensor itself must be kept in a germ-free condition. Further, when a number of food articles are continuously transferred on the conveyor of a continuous freezing apparatus and so forth, it is impossible to measure the temperature of the food articles individually with thermistor and so forth.
An invention which, in a heat source apparatus for hydrous food, using as temperature detecting means a means for detecting impedance which varies with the temperature of the food due to the change of conductivity and permittivity of the same placed on a pair of electrodes formed on a base plate, performs chilled/partial control of the heat source apparatus, based on the detected value, is disclosed in Japanese Patent Publication No. 7-76664.
In the invention mentioned above, impedance is measured, placing a food on the pair of electrodes, but the state of contact of the food with the electrodes and the contact resistance are not constant and the measurement is always performed in an unstable condition and so the measurement value is inaccurate, therefore, the control of the heat source apparatus with high accuracy based on the measurement value can not be expected.
Conventionally, to grasp the various states of a food article, a variety of sensors are used, but with positions of sensors fixed, the temperatures of the food article can not be measured accurately when food articles of various size are transferred in a continuous freezing apparatus.
The present invention was made in light of such problems as mentioned above, and inventors took notice of the fact that the change in physical properties and composition of a food due to bonding/separation of water, change in molecular structure, and etc., caused by the freezing and unfreezing of the food, causes a change in permittivity of the food and the change in the permittivity correlates with the change in the capacitance between a pair of electrodes holding the food article in between without contact with them.
It is thinkable that the temperature of a food article in a frozen or unfrozen state correlates with the capacitance between the electrodes, and particularly at freeze point at which the physical properties of food change largely, change in the electrostatic capacity occurs owing to the change in physical properties of the food in accordance with the temperature of the same.
The object of the present invention is to provide a non-contact article temperature measuring device applicable to a freezing/unfreezing system which enables the evaluation of quality including the food temperature, frozen/thawed state, and presence/absence of bubble, and also enables the operation control of the unfreezing apparatus.